Selenium (Se) has been thought to hold great promise as a cancer chemopreventive agent. However, the negative efficacy outcomes of recently concluded human trials in North America with seleno-methionine (SeMet) for prostate cancer (PCa) prevention (SELECT, HGPIN trials) have caused many to conclude that there is no hope for using Se to prevent cancer. We, however, beg to disagree with such a mainstream opinion. A costly lesson from these well-executed clinical studies is that whatever preclinical efficacy data that existed at the time the decision was made to conduct these trials DID NOT support the choice of SeMet for human efficacy validation, NOR did cell culture-based studies. In contrast, many pre-clinical studies from our group and others have shown inhibition of the genesis of cancers of the prostate and other organ sites by supra-nutritional intake of other Se forms, especially methylseleninic acid (MSeA) and Se- methylselenocysteine (MSeC), the presumed precursors of in vivo active anti-cancer methylselenol pool. Our recent data with a proteomic approach question the validity of the methylselenol paradigm and suggest unique potential molecular targets for MSeA vs. MSeC with little overlap. Our latest data with organ specific Pten- knockout (KO) driven prostate carcinogenesis suggest a super-activation of p53-p21-senescence pathway by MSeA to suppress neoplastic prostatic lesion growth in a preclinical mouse model, heralding in a new prevention paradigm through inducing irreversible terminal arrest of early lesion cells in vivo. Our central hypothesis is that MSeA/C will prevent prostate carcinogenesis in preclinical rodent models each by regulating distinct sets of molecular targets and signaling pathways (e.g., p53-p21-senescence; p53- apoptosis; suppressor/oncoprotein balances). We propose testing this hypothesis with 3 Specific Aims: 1. To contrast the in vivo preventive efficacy of MSeA/C with (the lack thereof) SeMet against epithelial adeno- carcinogenesis in the prostate specific Pten-KO mice and to critically assess the contribution of the p53-p21 senescence pathway to their efficacy using Pten and p53 double KO mice. 2. To contrast the in vivo preventive efficacy of MSeA/C with the lack thereof by SeMet against chemically-induced, androgen-promoted prostate carcinogenesis in rats and to determine whether p53-p21 senescence activation is involved. 3. To identify proteomic signatures and molecular targets in the prostate gland and prostate carcinomas from studies of Aims 1 and 2 of MSeA and MSeC in addition to the p53-p21-senescene pathway and validate identified key targets. The results are expected to provide solid in vivo efficacy data for MSeA/C from multiple preclinical models of prostate carcinogenesis representing diverse human PCa etiologies and different mammalian species (mice and rats). Positive efficacies in multiple models increase the translatability for human application. The results may help to resurrect and advance the field of Se-prostate cancer chemoprevention research, redefining and changing mechanism paradigms of active Se form(s), cellular processes and molecular targets.